Various types have been known for a flowmeter [flow rate sensor] (or current meter [flow velocity sensor]) for measuring the flow rate (or flow velocity) of various kinds of fluid, particularly liquid. Of these types of flowmeters, a so-called thermal type (particularly, indirectly heated type) flowmeter has been used because the price thereof is lower.
One of indirectly heated type flowmeters is designed and used so that a sensor chip comprising a thin-film heating element and a thin-film temperature sensing element which are laminated on a substrate through an insulating layer by using the thin film technique is disposed so as to enable the heat transfer between the sensor chip and fluid flowing in a pipe. The electrical characteristic of the temperature sensing element, for example, the value of the electrical resistance is varied by supplying current to the heating element to heat the temperature sensing element. The variation of the electrical resistance value (based on increase of the temperature of the temperature sensing element) is varied in accordance with the flow rate (flow velocity) of the fluid flowing in the pipe. This is because a part of the heating value of the heating element is transferred into the fluid, the heating value thus diffused into the fluid is varied in accordance with the flow rate (flow velocity), and the heating value supplied to the temperature sensing element is varied in accordance with the variation of the heating value diffused into the fluid, so that the electrical resistance value of the temperature sensing element is varied. The variation of the electrical resistance value of the temperature sensing element is also varied in accordance with the temperature of the fluid. Therefore, a temperature sensing element for temperature compensation is installed in an electrical circuit for measuring the variation of the electrical resistance value of the temperature sensing element to reduce the variation of the flow rate measurement value due to the temperature of the fluid as much as possible.
With respect to the indirectly heated type flowmeter using the thin-film element as described above, JP(A)-11-118566 discloses an example of the indirectly heated type flowmeter. The flowmeter disclosed in this publication uses an electrical circuit (detection circuit) containing a bridge circuit to achieve the electrical output corresponding to a flow rate of fluid.
It is general that the output of the electrical circuit of the flowmeter is not in a simply proportional relationship with the flow rate value, and variation of the output of the electrical circuit to the flow rate variation is large in an area where the flow rate value is small while the variation of the output of the electrical circuit to the flow rate variation is small in an area where the flow rate value is large. Therefore, there is a problem that even when little error occurs on measured flow rate values due to the variation of the output of the electrical circuit in the small flow rate value area, the error is increased in the large flow rate value area (that is, the rate of the flow rate difference to be discriminable when the measurement is carried out is increased).
In order to avoid this problem, it has been hitherto general that a flowmeter is prepared for each relatively narrow flow rate range and the characteristic value of the electrical circuit is properly set every flow rate range. Therefore, if attention is paid to each individual flowmeter, it has a problem that the dynamic range of the flow rate measurement is small and the application of the indirectly heated type flowmeter suffers restriction.
Therefore, an object of the present invention is to provide an indirectly heated type flowmeter which can perform a flow rate measurement with excellent precision over a broad flow rate range.
In the flowmeter disclosed in JP(A)-11-118566, the voltage to be applied to the heating element is varied in accordance with the variation of the flow rate to thereby vary the heating state of the heating element so that the temperature sensing element is kept to a predetermined temperature (heating state), and the flow rate value is achieved on the basis of the voltage applied to the heating element at this time.
The environmental temperature at which the flowmeter is used is broad. For example, when the flowmeter is used in a cold district, the temperature of the flowmeter may be kept under 5° C. or less. On the other hand, when the flowmeter is used in a hot district, the temperature of the flowmeter may be kept at 35° C. or more. Even when the flowmeter is used in the same district, the environmental temperature of the flowmeter is varied in accordance with day and night. Accordingly, when the flow rate value is achieved on the basis of the voltage to be applied to the heating element as described above, there is a problem that the measurement value is varied in accordance with the environmental temperature, which is caused by variation of the characteristic of the electrical circuit of the flowmeter due to the temperature variation.
The present invention has another object to improve the control of an applied voltage to the heating element in the indirectly heated type flowmeter as described above and achieve high precision and high control response without complicating the circuit construction.
Further, the present invention has another object to prevent the variation of the measurement value due to the environmental temperature in the indirectly heated type flowmeter as described above, and further enhance the precision of the flowmeter.
When the flow rate detection is carried out by using the thermal type flowmeter, the following problems occur due to the variation of the temperature of fluid for which the flow rate is detected.
For example, in a case where a kerosene flow passage is formed by a pipe so as to extend from a kerosene tank disposed outdoors to kerosene burning equipment disposed indoors and a flowmeter is disposed in an indoor portion of the pipe, if there is a large difference between the outdoor temperature and the indoor temperature (for example, the difference in temperature may be equal to about 20° C. in the winter season), kerosene remaining in the indoor portion of the pipe first passes through the flowmeter at the initial stage where use of the kerosene burning equipment is started, and after some amount of kerosene passes through the flowmeter, kerosene existing in the outdoor portion of the pipe at the initial stage reaches the flowmeter to be detected in flow rate.
In most cases, a fluid temperature detecting unit containing a temperature sensing element for temperature compensation installed in a fluid flow rate detecting circuit is disposed at a position different from that of a fluid flow rate detecting unit, or even when they are disposed to be near to each other, a heat-exchange portion of the fluid flow rate detecting unit at which heat exchange is actually carried out to detect the fluid flow rate is far away from a heat-exchange portion of the fluid temperature detecting unit at which heat exchange is actually carried out to detect the fluid temperature. In these cases., if fluid which quickly varies in temperature flows into the flowmeter as described above, there appears temporarily such a state that the fluid temperature when the heat-exchange with the fluid temperature detecting unit is carried out is different from the fluid temperature when the heat-exchange with the fluid flow rate detecting unit is carried out. Therefore, accurate temperature compensation cannot be performed and thus the precision of the fluid flow rate detection is reduced.
Therefore, the present invention has an object to provide a flowmeter which can accurately make a fluid temperature compensation and thus perform accurate flow rate detection even when the temperature of fluid flowing into the flowmeter quickly varies.